Method for fractional crystallisation of a metal

ABSTRACT

The invention relates to a method for fractional crystallization of an at most partially solidified molten metal, in which a layer of at most partially solidified molten metal having an upper surface and a lower surface is divided into a series of compartments communicating with each other, in which the metal is stirred in at least some of the compartments, and in which crystals formed and/or existing in the layer of metal are selectively transported in a predetermined direction and molten metal is selectively transported in the opposite direction.

This is the U.S. National Stage of Patent Cooperation Treaty applicationNo. PCT/EP2003/006902 having an international filing date of 27 Jun.2003 and claiming priority from European patent application No.02077683.7 filed 5 Jul. 2002. The invention relates to a method forfractional crystallisation of a molten metal.

Crystallisation methods and apparatus are used to refine a metal (hereused as an abbreviation for metal alloy) in which too high aconcentration of a foreign element is present. This foreign element canbe present because in the metal made from metal ore, the primary metal,too much of the foreign element is present, or because already usedmetal is recycled and the foreign element concentration in the scrap istoo high. For instance aluminum scrap can contain too much of theforeign elements Fe, Si or Mg for use for commercial purposes withoutmixing it with primary metal containing little of the foreign element.

When use is made of fractional crystallisation to refine the metal,crystals are formed in the molten metal during partial solidification ofthe molten metal, which crystals have a composition that is differentfrom the composition of the molten metal that is used as a startingpoint.

The crystals formed in the molten metal during partial crystallisationof the molten metal are for instance refined as compared to the moltenmetal. The amount of refinement however depends on the type of theforeign element and the type of metal, and also on the quantity offoreign element present in the molten metal used as a starting point.One refinement step often is not enough to get a refined metal that ispure enough for commercial purposes.

It is an object of the invention to provide a method for fractionalcrystallisation of a metal with which a highly refined metal can beproduced.

It is another object of the invention to provide a relatively simplemethod to refine metals by fractional crystallisation.

It is still another object of the invention to provide a method that canbe used with a relatively simple apparatus.

It is a further object of the invention to provide such a method forcontinuous fractional crystallisation.

One or more of these objects are reached with a method for fractionalcrystallisation of an at most partially solidified molten metal, inwhich a layer of at most partially solidified molten metal having anupper surface and a lower surface is divided into a series ofcompartments communicating with each other, in which the metal isstirred in at least some of the compartments, and in which crystalsformed and/or existing in the layer of metal are selectively transportedin a predetermined direction and molten metal is selectively transportedin the opposite direction.

Due to the presence of compartments in the layer of metal and thestirring of the metal in the compartments, the crystals formed and/orexisting in the layer of metal remain in their compartment for sometime. The stirring of the metal also results in the presence of a thinboundary layer around the crystals. Since the compartments communicatewith each other, some of the crystals will be transported from onecompartment to the other as a result of the selective transportation inthe predetermined direction. In each compartment a further refinementcan take place, resulting in highly refined crystals in the lastcompartment. To produce metal with a refinement that is that high,molten metal that is less refined must be transported in the directionopposite or counter current to the transportation of the crystals. Theoverall refinement and the yield of the method depend on the amount ofmolten metal in counter current.

Preferably a temperature difference is present over the length of thelayer of metal, the higher temperature being present at the end of themetal layer to which the crystals are transported.

As a result of the temperature difference over the length of the layer,a relatively small temperature difference will exist between adjacentcompartments. The temperature in the compartment to which the crystalsare selectively transported will thus be higher than the temperature inthe compartment from which the crystals are transported. In thecompartment having the higher temperature the molten metal will be morerefined. The crystals formed in this compartment will therefore be morerefined than the former crystals. In each compartment in the directioninto which the crystals are transported, the crystals will thus be morerefined. Highly refined crystals and molten metal will thus exist at theend of the series of compartments in the predetermined direction oftransport of the crystals. To form crystals in the layer of metal, thelayer of metal has to be cooled in some way.

According to a preferred method the compartments in the layer of metalare formed by compartment walls that are present in pairs, thecompartment walls of each pair being preferably placed adjacent to eachother, one wall extending towards and adjacent to the lower surface ofthe layer of metal and the other wall extending from the lower surfaceof the layer of metal towards the upper surface of the layer of metal.

In this way crystals that are heavier than the molten metal will sink inthe compartment, but due to the stirring of the metal some of thecrystals in the compartment will fall into the next compartment, overthe wall extending from the lower surface. The crystals cannot betransported in the other direction, so this measure results in aselective transport of the crystals in one direction. When crystals aretransported in one direction through the layer of metal, molten metalhas to flow in the opposite direction. If the compartment walls of apair of compartment walls are placed adjacent to each other, aspreferred, the molten metal has to flow between the pair of walls in thecounter direction of the crystals, and relatively little molten metalcan be transported with the crystals into the next compartment. In thisway the molten metal in each compartment is kept as refined as possible.

Alternatively, the compartments in the layer of metal are formed bycompartment walls that are present in pairs, the compartment walls ofeach pair being preferably placed adjacent to each other, one wallextending from the upper surface of the layer of metal towards the lowersurface of the layer of metal and the other wall extending towards andadjacent to the upper surface of the layer of metal. This alternativeshould be used when the crystals formed and/or existing in the moltenmetal are lighter than the molten metal. This method is used in ananalogous way.

The transport of crystals and molten metal between the pair ofcompartments walls can be optimised by the design and positioning ofthese walls. For example, the compartment walls can be placed at anangle relative to each other such that the distance between a pair ofcompartment walls increases in the direction of crystal transport. As aresult, the velocity of the molten metal in counter current decreasesand the velocity of the crystals increases between the pair ofcompartment walls, thus minimizing the risk of almost stationarycrystals blocking further crystal transport.

According to another preferred method a layer of transporting liquid ispresent below and/or above the layer of metal to selectively transportthe crystals, and the compartments in the layer of metal are formed bycompartment walls extending towards and adjacent to the layer oftransporting liquid transporting the crystals, preferably thetransporting liquid being a molten salt.

As in the former preferred method, the crystals formed and/or existingin the layer of metal are kept in suspension by the stirring. Some ofthe crystals will sink or rise towards a layer of transporting liquid,which layer of transporting liquid will transport the crystals towardsthe next compartment. As a result the selective transportation isachieved. Here too the molten metal is transported in counter currentand due to the compartment walls ending adjacent to the layer oftransporting liquid transporting the crystals, relatively little moltenmetal can be transported with the crystals into the next compartment. Amolten salt is preferably used as a transporting liquid because it willnot react with molten metal and can withstand high temperatures.

According to still another preferred method, the layer of metal ispresent in a chamber having an inclined bottom, and the compartments inthe layer of metal are formed by compartment walls extending towards andadjacent to the bottom of the chamber.

As above, the crystals are kept in suspension in the compartments due tothe stirring of the molten metal. Some of the crystals sink to theinclined bottom, and due to the gravity these crystals are transportedinto the next lower compartment. Here the selective transport isachieved by the inclination of the bottom, and due to the compartmentwalls ending adjacent to the bottom relatively little molten metal istransported with the crystals into the lower compartment, but the moltenmetal is transported in counter current.

As an alternative to this last preferred method, the layer of metal ispresent in a chamber having an inclined upper wall, and the compartmentsin the layer of metal are formed by compartment walls extending towardsand adjacent to the upper wall of the chamber. This alternative shouldbe used when the crystals are lighter than the molten metal and rise inthe compartments.

Preferably the compartment walls are adjustable such that the ends ofthe compartment walls are placed nearer to or further from the surfaceof the layer of metal they extend towards. In this way the transport ofcrystals can be restricted or increased, depending on the other processvariables, the type of metal, et cetera.

According to a preferred method mixing means are present to stir themetal in at least some of the compartments, the mixing velocity of themixing means being variable. The mixing velocity of the mixing means canbe used to control the transport of the crystals from one compartmentinto the next compartment.

Preferably, molten metal and/or crystals are removed at the end of thelayer of metal towards which the crystals are selectively transported.Here the refinement of the metal is highest. Of course unrefined moltenmetal has to be added and remaining molten metal, containing a higheramount of the foreign element, has to be removed.

Preferably the metal used is aluminum. Aluminum is one of the metalsthat are difficult and/or costly to refine by conventional methods. Themethod according to the invention is particularly suited for thefractional crystallisation of aluminum in a relatively easy and costeffective way.

The fractional crystallisation as described above is preferably used forremoving one or more of the elements Fe, Ga, Mg, Mn, B, Si, Sn, Zn andNi from the aluminum.

The invention will be elucidated referring to an exemplary embodiment,in view of the accompanying drawing.

FIG. 1 shows, in a schematic way, a cross section through acrystallisation apparatus for implementing the method according to theinvention.

FIG. 1 shows a portion of a crystallisation apparatus 1 for thecontinuous crystallisation of molten metal containing one or moreforeign elements, that is presently preferred for the purpose. Thecrystallisation apparatus has a chamber 2 with a bottom 3 and an upperwall 4, which are very well isolated as is known in the art, normally byspecial refractory materials.

In the chamber 2 compartments 5, 6, 7 are formed by compartment walls 8,9 that are present in pairs. The compartment walls 8 are attached to theupper wall 4 and end adjacent to the lower wall. It would however bepossible to place the high end of the compartment walls 8 some distancefrom the upper wall 4, dependant on the height of the chamber 2 and thetype of metal to be crystallised. The compartment walls 9 are attachedto the bottom 3 of the chamber 2 and end at a distance from the upperwall 4. The height of the compartment walls 9 depends on the metal to becrystallised and the process conditions during the crystallisation. Ineach compartment a mixing element 10 is present to stir the molten metalwith the crystals formed and/or existing in the metal.

Only three compartments 5, 6, 7 are shown. It will be appreciated thatthe crystallisation apparatus will contain the number of compartmentsthat is necessary for the desired refinement of a certain metal,depending on the amount of foreign element(s) that is/are present asstarting point and the process conditions.

As always necessary for crystallisation the molten metal in thecrystallisation apparatus has to be cooled. Cooling means to do so havenot been shown but are known in the art.

The above described crystallisation apparatus can for instance be usedfor the continuous fractional crystallisation of aluminum containing0.10% Si and 0.20% Fe (so-called P1020) to reach aluminum containingless than 0.01% Si and 0.01% Fe (so-called P0101).

For this crystallisation process chamber 2 of the crystallisationapparatus 1 has to have eight to ten compartments, each compartmenthaving a size of approximately 500×500×500 mm³, so the chamber has aninner size of approximately 4 à 5 m (length)×0.5 m (width)×0.5 m(height). The compartment walls 8 end approximately 80 to 100 mm fromthe bottom 3 and the compartment walls 9 are approximately 400 mm high.The height of the compartment walls 9 will however be dependant on therotational velocity and the size of the mixing elements 10. The distancebetween the compartment walls 8 and 9 is approximately 80 mm.

The method according to the invention implemented for aluminum with theabove apparatus is as follows.

Molten aluminum with P1020 composition is introduced into the apparatusthrough an inlet (not shown) at a temperature just above thecrystallisation temperature of approximately 660° C. The molten aluminumin the chamber 2 is cooled using cooling means (not shown) to formcrystals. These crystals contain less of the foreign elements Si and Feand tend to slowly sink through the molten aluminum to the bottom 3.

The stirring action of the mixing elements 10 keeps the crystals insuspension. For instance in compartment 5 continuously some crystalsmove over the compartment wall 9 (see arrow A). This amount depends onthe size of the crystals, the rotational velocity of the mixing element10 and the height of the compartment wall 9. The crystals that move overthe compartment wall 9 sink to the bottom 3 in compartment 6 because inbetween the compartment walls 8 and 9 the molten metal is not stirred.Once the crystals have sunk below the lower end of compartment wall 8,they will be swept up by the stirring action of the mixing element 10 incompartment 6. The crystals in compartment 6 are kept in suspension bythe mixing element and continuously a certain amount of the crystalsmoves to compartment 7 in the same way.

In this way the crystals are selectively transported from the right-handend of the apparatus to the left-hand end. Since crystals aretransported to the left, molten metal has to move to the right throughthe apparatus. Because the distance between the compartment walls 8 and9 is small, here the molten metal only moves upwards and effectivelyonly crystals move downwards. In this way a counter current existsthrough the apparatus.

Over the length of the apparatus a temperature difference exists suchthat the temperature of the molten metal at the left-hand of theapparatus is higher than the temperature at the right-hand end of theapparatus as seen in FIG. 1. This means that for instance thetemperature in compartment 6 is slightly higher than in compartment 5. Acrystal formed or existing in compartment 5 will be more refined thanthe molten aluminum in which it is formed or present. When this crystalis transported to compartment 6 where the temperature is slightlyhigher, the crystal will partly or totally melt, which leads to acomposition of the molten aluminum in compartment 6 that is more refinedthan the molten aluminum in compartment 5. In compartment 6 crystalswill be formed again. The crystals formed in compartment 6 will thus bemore refined than the crystals in compartment 5.

The same holds for all neighbouring compartments in the crystallisationapparatus 1, thus leading to very refined aluminum in the left-handcompartment of the apparatus, where the refined aluminum can bedischarged. Due to the counter current aluminum containing a lot of Siand Fe can be discharged at the right-hand end of the apparatus of FIG.1 as a by-product.

For the control of the crystallisation, the apparatus is preferablyequipped with means to measure and control the solid fraction, thechemical composition and/or the temperature in the layer of metal.

With the above-described apparatus, a production of about 20 tons perday of aluminum with P0101 composition can be reached; the by-productwill only be some 10% thereof.

It will be understood that many changes can be made or will be necessarydepending on the metal used and the foreign element that has to beremoved from it. For instance, if the crystals that are formed and/orpresent rise in the molten metal the compartment walls have to be placedupside down. It will also be possible to use other means for stirringthe molten metal in the compartments.

On the other hand it will also be possible to use other types ofapparatus to effectuate the methods as described in the introduction tothe description. Thus, the scope of the invention will only bedetermined by the accompanying claims.

1. Method for fractional crystallisation of an at most partiallysolidified molten metal, comprising: introducing the at most partiallysolidified molten metal into a chamber with a lower wall and an upperwall and divided into a series of compartments communicating with eachother, wherein the introduced at most partially solidified molten metaloptionally comprises crystals, stirring the metal in at least some ofthe compartments, forming crystals in a layer of the metal in thecompartments, and wherein crystals in the layer of the metal in thecompartments are selectively transported in a predetermined directionand molten metal is selectively transported in the opposite direction;wherein the method refines aluminum, wherein a temperature difference ispresent over the length of the layer of metal, the higher temperaturebeing present at an end of the metal layer to which the crystals aretransported.
 2. Method according to claim 1, wherein the compartments inthe chamber are formed by compartment walls present in pairs, one wallextending towards and adjacent to the lower wall of the chamber and theother wall extending from the lower wall of the chamber towards theupper wall of the chamber.
 3. Method according to claim 2, wherein thecompartment walls are adjustable such that the ends of the compartmentwalls are placed nearer to or further from the upper wall and lowerwall, respectively, of the chamber.
 4. Method according to claim 2,wherein the compartment walls of each pair being preferably placedadjacent to each other.
 5. Method according to claim 1, wherein thecompartments in the chamber are formed by compartment walls present inpairs, one wall extending from the upper wall of the chamber towards thelower wall of the chamber and the other wall extending towards andadjacent to the upper wall of the chamber.
 6. Method according to claim5, wherein the compartment walls are adjustable such that the ends ofthe compartment walls are placed nearer to or further from a surface ofthe layer of metal they extend towards.
 7. Method according to claim 1,wherein a layer of transporting liquid is present below and/or above themetal to selectively transport the crystals, and the compartments in thechamber are formed by compartment walls extending towards and adjacentto the layer of transporting liquid.
 8. Method according to claim 1,wherein the lower wall of the chamber is inclined, and the compartmentsare formed by compartment walls extending towards and adjacent to thelower wall of the chamber.
 9. Method according to claim 1, wherein theupper wall is inclined, and the compartments are formed by compartmentwalls extending towards and adjacent to the upper wall of the chamber.10. Method according to claim 1, wherein mixing means are present tostir the metal in at least some of the compartments, the mixing velocityof the mixing means being variable.
 11. Method according to claim 1,wherein molten metal and/or crystals are removed at the end of the layerof metal towards which the crystals are selectively transported. 12.Method according to claim 1, wherein said method removes one or more ofthe elements Cu, Fe, Ga, Mg, Mn, B, Si, Sn, Zn, and Ni from thealuminum.
 13. Method according to claim 1, wherein the compartments areformed by compartment walls present in pairs, the compartment walls ofeach pair being placed adjacent to each other, one wall extending fromthe upper surface of the layer of metal towards the lower surface of thelayer of metal and the other wall extending towards and adjacent to theupper surface of the layer of metal.
 14. Method according to claim 1,wherein a layer of transporting liquid is present below and/or above thelayer of metal to selectively transport the crystals, and thecompartments are formed by compartment walls extending towards andadjacent to the layer of transporting liquid transporting the crystals,the transporting liquid being a molten salt.
 15. Method according toclaim 1, wherein the method removes Fe from the aluminum.
 16. Method forfractional crystallisation of an at most partially solidified moltenmetal, comprising: introducing the at most partially solidified moltenmetal into a chamber with a lower wall and an upper wall and dividedinto a series of compartments communicating with each other, wherein theintroduced at most partially solidified molten metal optionallycomprises crystals, stirring the metal in at least some of thecompartments, forming crystals in a layer of the metal in thecompartments, and wherein crystals in the layer of the metal in thecompartments are selectively transported in a predetermined directionand molten metal is selectively transported in the opposite direction;wherein the method refines aluminum, wherein a temperature difference ispresent over the length of the layer of metal, the higher temperaturebeing present at a first end of the chamber to which the crystals aretransported and a lower temperature being present at a second end of thechamber to which the molten metal is transported, wherein thetemperature in a first said compartment closer to the first end ishigher than a second said compartment relatively closer than the firstcompartment to the second end, wherein, the crystals formed and/orexisting in the layer of metal in at least one respective compartmentcomprise aluminum and Fe while the molten metal in said respectivecompartment comprises a lower aluminum content than the crystals in saidrespective compartment and a higher Fe-content than the crystals in saidrespective compartment.
 17. Method for fractional crystallisation of anat most partially solidified molten metal, comprising: dividing a layerof at most partially solidified molten metal having an upper surface anda lower surface into a series of compartments communicating with eachother, stirring the metal in at least some of the compartments, formingcrystals within the layer of metal, and wherein crystals in the layer ofmetal in the compartments are selectively transported in a predetermineddirection and molten metal is selectively transported in the oppositedirection, wherein a layer of transporting liquid is present belowand/or above the layer of metal to selectively transport the crystals,and the compartments in the layer of metal are formed by compartmentwalls extending towards and adjacent to the layer of transporting liquidtransporting the crystals, the transporting liquid being a molten salt.18. Method for fractional crystallisation of an at most partiallysolidified molten metal, comprising: introducing the at most partiallysolidified molten metal into a chamber with a lower wall and an upperwall and divided into a series of compartments communicating with eachother, wherein the introduced at most partially solidified molten metaloptionally comprises crystals, stirring the metal in at least some ofthe compartments, forming crystals in a layer of the metal in thecompartments, and wherein crystals in the layer of the metal in thecompartments are selectively transported in a predetermined directionand molten metal is selectively transported in the opposite direction;wherein a temperature difference is present over the length of the layerof metal, the higher temperature being present at an end of the metallayer to which the crystals are transported.